The ability to detect defects in optical lightguide fibers is critical in providing high quality fiber and in devising manufacturing techniques that minimize the occurrence of such defects. Defects or inhomogenieties can affect the strength or transmission characteristics of the optical fiber. One class of defects, loosely defined as "bubbles" or "air lines", can range from the sub-micron (.mu.m) to several microns in diameter and multiple-meter lengths. Although the term "air line" is used, defects can take on many different shapes and geometries. Defects over 10 .mu.m in diameter can cause a variety of problems, including proof test breaks in the manufacturing process and fiber splice problems in the installation process.
Techniques for detecting defects in fibers (and, incidentally, dealing with the effects of defects on fiber diameter measurements) are known. See, for example, U.S. Pat. No. 4,046,536, issued Sep. 6, 1977, to D. H. Smithgall (analysis of fringe counts in the presence of "dropouts" resulting from faults in the fiber); U.S. Pat. No. 4,501,492, issued Feb. 26, 1985, to N. Douklias (detection of fiber defects and testing of fiber diameters using a spatial filter prepared using diffracted/scattered light from a defect-free fiber); and U.S. Pat. No. 5,185,636, issued Feb. 9, 1993, to L. J. Butten, et. al. (detection of fiber defects using light scattered from a fiber diameter measurement unit and performing Fast-Fourier-Transform (FFT) to examining the spectrum).
Although these techniques can detect defects in optical fibers, they nonetheless have several significant limitations, including cost and complexity. The added cost and complexity of such methods are due, in large part, to computational requirements and expenses associated with analyzing the frequency spectrum of light scattered signals, for example, performing a FFT on the light scattered signal repeatedly.
With increased market competition and heightened customer expectations, it has become important to develop a low-cost method to detect defects in optical fiber as it is drawn in the manufacturing process and deal with such defects in the fiber accordingly.